A unit for joining a flat liner paper sheet to a corrugated paper sheet in corrugated board manufacturing machines, comprising:
a) at least one toothed or corrugated roll, which is mounted in such a way as to rotate about its axis and which drags a corrugated paper sheet in a position partly adhering to its perimetral surface. PA0 b) means for feeding and pressing at least one flat paper sheet, which interacts with the toothed or corrugated roll in laying the flat paper sheet over the corrugated paper sheet, conveyed by said toothed or corrugated roll; PA0 c) gluing means, arranged in such a way as to apply adhesive to one of the sheets to be joined at least before the latter are mutually compressed.
The fabrication of corrugated board substantially provides a step in which a paper sheet, or similar is corrugated and further, the superposition of a flat paper sheet thereto, which is anchored to the corrugated sheet by gluing and mutual compression using several known devices.
A first device provides means for feeding and pressing the flat paper sheet against the corrugated paper sheet, in the form of a pressure roll oriented parallel to the toothed and corrugated roll and rotatable in the counter-rotating direction, which pressure roll is tangent to the toothed or corrugated roll. The toothed or corrugated roll, at least its outer ring gear is generally made of steel, as well as the pressure roll. In this way, when each tooth comes into a tangent position with respect to the shell of the pressure roll, a compressing action is readily exerted on the paper sheets interposed therebetween, having a sort of striking effect, and involving a change in the appearance of the hard paper sheet which forms the exposed face of the product. In the jargon of the field, this mark is defined as glazing deformation of the paper surface or also bright line due to the brighter aspect of paper where the mark of the compressing action is.
An alternative system provides means for feeding and pressing the flat paper sheet against the corrugated paper sheet, gearing with the toothed or corrugated roll, in the form of a feeding/pressing belt, having a predetermined tension and following the shell surface of the toothed or corrugated roll for a certain angular width, while being in contact therewith, and being complementarily deformed, the two paper sheets being interposed therebetween. The feeding/pressing belt exerts a more uniform compression action on the two sheets avoiding, thanks to a certain elasticity and compliance, the localized peaks of pressure caused by the teeth or corrugations of the toothed or corrugated roll. However, in this case, while no buckling effect nor surface mark is produced on the flat sheet, the two sheets are often feebly glued, thus causing them to be easily separated.